This invention relates to a jig unit for heat treatment of a vessel-like workpiece having an inner concave surface such as, for example, a hollow hemispherical part of a rocket motor case and a heat treatment method using the same jig unit.
In heat treatment of vessel-like metal workpieces which are relatively large in size and relatively small in wall thickness, special car should be taken to prevent or minimize defects such as soft spots and quenching straings. Examples of vessel-like workpieces having inner concave surfaces of concern include hollow cylindrical, conical or hemispherical sections of solid fuel rocket motor cases. Greater care is needed when quenching the heated vessel-like workpieces, because the cooling of a workpiece may be locally delayed in a region remote from the open end of the workpiece by the influence of hot air as well as, a vapor that originates from the cooling liquid which vapor is confined in the interior of the vessel-like workpiece. This is particularly important in the case of a hollow conical or hemispherical workpiece which must be held with its top or polar region upward during the heating and cooling operations.
A known support unit for heat treatment of, for example, a hollow hemispherical workpiece is comprised of an annular bed on which the work is placed its open end down and an exhaust duct having an end section which stands in the center of the annular bed to discharge gases from the interior of the hemispherical workpiece. The support unit is transferrable. After fastening the workpiece to the annular bed the support unit is placed in a furnace for heating the workpiece. After completion of the heating operation the support unit holding the heated workpiece is immersed in a cooling liquid. At this stage the exhaust duct serves the purpose of discharging air and the vapor of the cooling liquid from the interior of the hollow workpiece. By using such a support unit it is possible to decrease defects such as soft spots and quenching strains and accomplish almost uniform heat treatment of the hollow hemispherical workpiece since discharging air and vapor from the interior of the workpiece is effective for preventing delayed cooling of the closed end region of the workpiece.
However, still there are some problems. First, embrittled layers represented by oxide layers are formed on both the outer and inner surfaces of the vessel-like workpiece since the both surfaces are exposed to the heating atmosphere in the furnace for heat treatment. Removal of such embrittled layers, and particularly of the one on the inner surface of the vessel-like workpiece is a troublesome operation. Furthermore, quenching strains are liable to be produced in the major region of the vessel-like work, especially when the work is a large-sized one, since the work is heated and cooled in an unconstrained state except in its open end region clamped to the annular bed of the support unit. Therefore, the initial wall thickness of the work needs to be large enough for removal of the embrittled and strained layers by machining after the heat treatment. Then a large amount of grinding and/or milling is required for finishing the heat-treated work into a defectless and accurately sized part. Consequently the material and labor costs become considerably high.